1. Field of the Invention
This invention relates to a decoding apparatus for digital signal, particularly suited for decoding reproduction data of a CD-ROM on which sequential data is recorded.
2. Description of the Prior Art
A CD-ROM for recording digital data on an optical disc of a shape similar to a compact disc for music has been attracting attention as a recording medium for large-capacity data. The CD-ROM is suitable for real-time application programs which operate sequential data such as music data or picture data, since CD-ROM can record such long-time sequential data.
For this reason, the standards for recording music data or picture data using a CD-ROM, for example, the CD-I and the CD-ROM XA have been proposed. With such recording of music data or picture data onto the CD-ROM, man-machine interface is improved using a voice or an animated picture, and voice mail or picture mail using computer can be achieved. Besides, the realization of various services using the CD-I or CD-ROM has been expected.
For handling CD-ROM application programs, a CD-ROM disc driver and a CD-ROM decoder for reproducing the CD-ROM are needed, in addition to a host computer.
The CD-ROM disc driver capable of using real-time application programs is conventionally constructed so that it is connected to the CD-ROM decoder using a special standard interface. This is because to make strictly equal the speed of reproduced data and the speed of decoding data is needed in the real-time application.
In recent years, the SCSI (Small Computer System Interface) is frequently employed for the connection between a computer and peripheral equipment. Based on this background, in the market, there is strong demand for the connection of a CD-ROM disc driver and a CD-ROM decoder using the SCSI bus.
However, in principle, the SCSI bus cannot transmit a signal corresponding to the read-out speed of reproduced data from a recording medium. Therefore, it is difficult to make the reproduction data speed equal to the decoding speed strictly. For this reason, it is thought that the SCSI bus cannot connect the CD-ROM disc driver and the CD-ROM decoder conventionally.
Namely, when a signal corresponding to the speed of reproduction data cannot be transmitted, the CD-ROM disc driver and the CD-ROM decoder operate with their own clocks. In the case where the CD-ROM disc driver and the CD-ROM decoder are operated by their own clocks, it is impossible to make completely equal the playback time of the CD-ROM and the decoding time of the CD-ROM decoder.
Data is continuously reproduced from the CD-ROM, and the continuous data is decoded from the CD-ROM decoder. Consequently, even if there is a slight speed error between the speed of reproduced data and the decoding speed, the error is accumulated after long hours of playback.
As a result, when a CD-ROM which has recorded music data and picture data is played back, sound leaping or a time difference between the reproduced picture and the reproduced sound may take place.
For example, as shown in FIG. 1A, it is assumed that a disc which has recorded compressed audio data A and graphic data G is reproduced. At this time if the speed of reproduced data and the decoding speed coincide with each other, a reproduced and time-base expanded audio signal a coincides with a corresponding reproduced picture as shown in FIG. 1B.
On the other hand, FIG. 2 shows a case where there is a speed error between the reproduced data speed and the decoding speed. As indicated in FIG. 2A, when the disc which has recorded the compressed audio data A and the graphic data G is reproduced, the reproduced and time-base expanded audio signal a does not coincide with the corresponding reproduced picture as shown in FIG. 2B if there is an error between the reproduced data speed and the decoding speed. The error e is accumulated with the advance of the playback time.